Method for producing a solar module with thin-film solar cells which are series-connected in an integrated manner and solar modules produced according to the method, especially using concentrator modules

ABSTRACT

Known methods of producing large-surface integrated thin-film solar modules with an amorphous, poly- or microcrystalline absorber layer always comprise division and conversion structuring processes which can cause instabilities in the structuring and which are relatively expensive. According to the inventive method which can be used to fabricate substrate solar cells and superstrate solar cells, the mask which is used provides for structuring itself during the deposition of layers for the rear electrode and the absorber layer through its geometrical form. The use of a mask which can be reused as an independent element after use in this method allows for a relatively free range of possible geometric forms. This also makes possible applications inside and outside of buildings, including in the area of a window, from an esthetic and informal point of view. These types of application are also supported by the possibility of a structural connection between the solar modules produced according to the inventive method and light-collecting concentrator modules, in order considerably to increase their average and total energy conversion efficiency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of fabricating a solar module withstructured and integrated series-connected thin-film solar cells and tosolar modules made by the method. The support layer of such solar cellsmay be either a substrate or a superstrate.

Thin-film solar cells of either type are provided with light-absorbingabsorber layers of cost-efficient amorphous, poly- or micro-crystallinesemiconductor materials which may be precipitated or built up onlarge-surface substrates or superstrates by a many different methods.The small layer thickness of the absorber layers and the possibility ofproviding a structure during fabrication further reduce themanufacturing costs, so that thin-film solar cells constitute acost-efficient alternative to the cost-intensive silicon solar cellsmostly used at present and which as mono-crystalline single or multiplelayer systems following their manufacture must first be sawed apart intoindividual cells and then, in the manner of high-value semiconductorproducts, be further processed by complex steps. By photovoltaicconversion of solar energy into electric power, thin-film solar cellsgenerate voltage levels of less than 1 Volt. In order to attaintechnically useful power at a voltage of typically 12 Volts or 24 Volts,a sufficient number of individual cells are connected in series. In thecase of thin-film solar cells, the series connection may be integratedinto the layer-forming process. This involves subdividing layers formedas whole surfaces into small strips by suitable structuring methods, forinstance paste-scribing methods and lift-off techniques as well asmechanical and, more particularly, layer processing methods. The purposeof the structuring is to create an electrical connection between theelectrodes at the front and rear surfaces of adjacent strip-shaped solarcells.

2. The Prior Art

From U.S. Pat. No. 4,675,467 there is known a method ofseries-connecting an integrated thin-film solar module in which bothelectrodes are incorporated in prefabricated strip-form into anunstructured absorber layer. The conductive connections between thecorresponding electrodes of adjacent solar cells are then formed in astructuring step by laser irradiation from the transparent substratesurface into an area covered by the electrode strips. Appropriate areasof the absorber layer are thus converted into low-ohmic areas by aprecisely defined quantity of energy, which does, however, entail therisk of damaging the semiconductor material. Because of the lack ofspatial structuring of the absorber layer there is no electricalinsulation between the semiconductor material of adjacent solar cellswhich leads to power-reducing short circuit currents. The lasertreatment requires a highly precise power level, positioning andfocusing of the applied laser beam in order to yield the desiredspatially precise conversion effect. This may lead to layer separationsand damages in the immediate vicinity of the structuring operation.Furthermore, it is necessary always to use a transparent substrate ofprecisely defined homogeneous layer thickness so that the laser beam maypenetrate from the substrate side and that a power-level dependent depthof penetration into the layers to be separated or transformed may beprecisely set.

A similar process involving prefabricated electrode strips has beendescribed in U.S. Pat. No. 4,999,308, in which the laser treatment forarea conversion is used at the same time to separate the absorber layerto form insulation trenches by blowing away semiconductor material whichis thus lost. In this combined process, the energy dosaging poses aproblem which to some extent leads to an uncertainty especially inrespect of the conversion areas, notwithstanding the fact thatprocessing is carried out from the upper side of the solar cells ratherthan through the substrate. The use of two scribing processes for theconsecutive separation of absorber layer and front electrode atdifferent laterally displaced positions is known from U.S. Pat. No.5,296,674. The separation is accomplished by indirect laser irradiationthrough the protective layer substrate so that the direct connectionbetween adjacent solar cells is maintained by the absorber layer. Thismethod requires multiple positioning of a transparent substrate whileaccepting short circuit currents.

A method of series connecting an integrated thin-film solar cell moduleis known from WO-9503628 in which all functional layers are separatelystructured during special process steps. In accordance with the method,a metal layer previously precipitated as a single layer on a transparentsubstrate is separated by any desired structuring method into closelyadjacent strips to form a strip-shaped rear electrode. Following thesubsequent whole-surface coating with a thin semiconductor layer forforming an absorber layer and with a front layer to form a frontelectrode, two additional separate structuring steps are carried out bylaser irradiation from the side of the substrate. The first laserirradiation serves to structure or pattern the strips of the absorberlayer and of the front electrode; the second laser irradiation serves toconvert into a low-ohmic area that portion of the absorber layer whichis positioned in the covered area between opposite electrode strips ofadjacent solar cells, thus forming an integrated conductiveseries-connection between the solar cells. Therefore, in the knownmethod the structuring involves a treble separation treatment. The firsttreatment serves to separate the front electrode and the absorber layer.Laser-assisted removal of the sensitive semiconductor layer alwaysinvolves the risk of damage to, or alteration of, the layer. The secondtreatment for area conversion again requires a precisely energized laserand includes the problems described supra.

The described methods are footed on the common task of optimization inthe sense of maximized power output or minimized surface area of thefabricated solar modules of striped structured thin-film solar cells ofeither type of support layers. Compared to monocrystalline solar cellssuch solar cells are of low energy efficiency which at lower lightconditions is rapidly further reduced relative to normal conditions(light concentration AM 1.5). Accordingly, at common deviations ofseasonal and day-by-day light intensities, depending upon the weatherand at indoor applications (down to 10% of the maximum availableradiation), thin-film solar cells are subject to significant powerlosses. This explains the insignificant use of thin-film solar cells inareas where solar radiation varies significantly and in indoor areas ingeneral. In connection with monocrystalline solar cells, lightcollecting concentrator modules consisting of optical elements tomaintain at, or raise to, an optimum efficiency range the lightintensity in respect of monocrystalline solar cells have become knownfrom many publications. However, the goal of such known measures is notto maximize power but, rather, to bring about as significant a reductionas possible of the very expensive required surface of the solar module.

For instance, U.S. Pat. No. 5,118,361 discloses a solar module ofmonocrystalline tandem solar cells made of GeAs/GaSb which is built intoa housing the cover of which is formed by a concentrator module made upof individual Fresnel lenses of polymeric material and positioned,together with light-collecting funnels, in front of individual solarcells. These are disposed within the module on a flexible connectionribbon of conductive and non-conductive strips. From European Patent0,657,048 there is known an automated microchip-like connection in avery similar arrangement of monocrystalline GaAs solar cells aiming atsurface minimization. Concentrator arrangements provided with linearfocus lenses in, or as part of, a module cover which are particularlysuitable for strip-like solar modules are known, for instance, from U.S.Pat. No. 4,711,972 for monocrystalline silicon solar cells and from U.S.Pat. No. 5,505,789 for monocrystalline integrated solar cell chips madeof GaAs. German Patent specification 197 44 840 A1 discloses a solarmodule having a forward concentrator module made of plastic Fresnellenses which as a structured unit tilts or slides in accordance with theposition of the sun to provide for an improved power balance. Finally,European Patent 0,328,053 describes strip-like solar modules with aforward Fresnel lens which are mounted in the corner of a window pane ofa double window and which are to provide the electricity for operatingblinds in the between the windows.

However, neither the mentioned nor any other publication discloses anapplication for concentrator modules for amorphous, poly- ormicro-crystalline thin-film solar modules in particular in any kind ofarrangement, so that such modules have hitherto shown a relatively poorpower balance which is strongly dependent on the time of day and on theweather. More particularly, little or no consideration has been given tooptical structuring methods in connection with applications of knownsolar modules, including thin-film solar cells precipitated as largesurfaces on glass substrates, which has led to relatively uniform solarmodule structures used primarily in the field of industry. Estheticallypatterned solar modules may be found, for instance, in roof pans (seeGerman Patents 42 279 29 and 43 176 74) or wrist watches which also maytake different colors into consideration (see European Patent0,895,141).

OBJECTS OF THE INVENTION

With reference to the above explanations and proceeding from the stateof the art relating to the method of fabricating a solar module withstructured and integrated series-connected thin-film solar cells whichis most closely related to the inventive method, it is a first object ofthe present invention to provide an improved fabrication method of verylow complexity as regards structuring. In this connection, those aspectsof the improvement addressed to the fabrication of substrate andsuperstrate cells alike are to relate especially to a simplified methodand, at the same time, to a fabrication process which can be more easilycontrolled and reproduced. Moreover, it is an object of the invention toachieve an exceptionally low use of material and an assured separationof the individual layers at a complete insulation of the individualsolar cells. Based on these consideration, the cost efficiency of thefabrication method is to be improved. In addition, the basic task ofoptimizing a solar module is to be accomplished by suitable forms andmeasures, especially in connection with concentrator modules of solarmodules fabricated in accordance with the inventive method, thus leadingto a special flexibility in terms of their applicability.

SUMMARY OF THE INVENTION

In the accomplishment of these objects, the invention provides forthin-film solar cells of the substrate type and for thin-film solarcells of the superstrate type. In this connection, the methods offabricating either type substantially correspond to each other, exceptthat they are quasi inversed. This may be explained by the fact that butfor their inversed layer sequence substrate and superstrate solar cellsare of the same basic construction. In this connection, the substratefunctions as a bottom support layer, and light impinges on the solarcell from above, whereas the superstrate functions as a top supportlayer, and light impinges through the superstrate. However, in order toprovide for a comprehensible description, the advantages of theinvention will primarily be described on the basis of the method offabricating a substrate type solar cell, particularly since theadvantages also result from the inverse method. Following this, thedifferences between the two methods will be briefly set forth.

The inventive method makes possible continuous structuring of allfunctional layers at an extremely low complexity of pattern formation.By using a mask, particularly one with a striped pattern, twostructuring steps otherwise required are avoided. On the one hand, thisrelates to the usual structuring of the rear electrode after applicationon the substrate or, in the inverse method, after application of theabsorber layer. In accordance with the invention, the rear electrode isstructured directly with a superposed mask during coating with acorresponding metal layer. This leads to no loss of material since themask may be used again. On the other hand, the method step of theotherwise conventional and particularly critical subsequent structuringof the semiconductor absorber layer is avoided as well. This is ofparticular advantage as problems arising during mechanical or, moreparticularly, laser-assisted cutting of the absorber layer are avoided.Damage to individual layers, imprecise borders between layers andnon-reproducible layer conditions resulting from laser irradiationscannot occur.

The absorber layer like the rear electrode is structured by use of themask. This leads to the integrated series connection of the individualsolar cells by a simple but especially effective method step. Beforeapplication of the absorber layer on the substrate and mask, the mask ismoved slightly in a lateral direction leading to the formation of narrowupper and lower ribs which after removal of the mask which are contactedby the front layer subsequently applied as an uninterrupted surface, thefront layer being the front electrode. In the area of the lower ribs therear electrode sections at one side are completely enclosed by theapplication of the absorber layer. Interruption of the absorber layerbetween the individual solar cells is achieved by the mask which iscovered as well so that no short circuit currents can occur. A void inthe absorber layer above the rear electrode sections on its other sideis achieved in the area of the upper ribs and serves for subsequentcontacting by the front electrode. Covering the mask at the same timeagain leads to no loss of material, at the same time the mask too iscompletely processed up to the absorber layer. After removal of the maskand application of the transparent conductive front layer as anuninterrupted surface the method in accordance with the inventionrequires only a single subsequent structuring step by mechanical orlaser assisted processes. At an interface which includes the upper ribsand a portion of any abutting solar cell, the front layer is simplyseparated down to the rear electrode at such a separation width as toform a correspondingly structured front electrode without short circuitsbetween individual solar cells. Difficult conversion processes in theabsorber layer for forming conductive ribs are avoided. The location ofthe interfaces is not particularly critical as they need be positionedlaterally displaced only in the area of the upper ribs or in thedirection of the abutting solar cell. As regards the position of theinterfaces it is important to prevent short circuits between the rearand front electrodes. When cutting in the area of the active solar cellthis is can be ensured.

The method of fabricating substrate solar cells in principle correspondsto the method of fabricating superstrate solar cells with the exceptionthat the method steps have to be carried out in reverse sequence.However, since the in the case of superstrate cells the transparentconductive front layer for the front electrode on the superstrate ismechanically not as sturdy as the metal layer for forming the rearelectrode on the substrate of substrate cells, the mechanical separationstep of structuring the front electrode cannot be carried out in thesame manner (in this case the scribing would penetrate down to thesuperstrate). Therefore, in the case of the method of claim 2 the entiretransparent superstrate is first covered with a conductive front layer.After the mask has been fixed, structuring is carried out by scribingalong the filled surface of the mask in a manner similar to using aruler so that the mask with one of its lateral margins is positioneddirectly adjacent to the structuring trenches. Thereafter, the absorberlayer is applied and the structuring trenches are also filled withabsorber material. Lateral sliding the mask again results in theformation of corresponding upper and lower ribs. Following applicationof the metal layer for structuring the rear electrode which may also bestructured as p-TLO, the mask which by now is also covered by a completesolar cell structure is finally removed. Therefore, the main differencefrom the method sequence of fabricating a substrate type solar cellresides in the advanced structuring step and the later removal of themask.

An important aspect of the inventive fabrication method resides in thedesigning of the mask. Whereas hitherto series-connected solar cellshave been structured continuously in stripes without alterations, theinvention makes it possible to introduce different designs. Almost anygeometric structure, for instance zig-zag or undulating patterns as wellas signatures or company logos, may be practiced by adherence to twosimple marginal conditions. These are, on the one hand, the basiccharacter of the structure which must be composed of individual smallerpartial surfaces. Such “digitizing” of analog structures does not,however, limit the optical function, as from a certain distance it is nolonger visually distinguishable if it is not a deliberately appliedelement of the geometric pattern. When segregating the structure intosmall partial surfaces their size, as the second condition, must becalculated such that any blank surfaces and, if applicable, any filledsurfaces in the geometric pattern must be of approximately the samesize. In this manner, any current irregularities are avoided which wouldoccur in case of partial surfaces of different sizes. Uniformity of thefilled surfaces is always required whenever—as set forth infra—the mask,too, is to be further processed into a complete solar cell. However, thesize of the filled surfaces may be different from that of the blanksurfaces. The second marginal condition does not pose any significantproblem in its realization as it may simply be incorporated into thearrangement of the solar module.

The possibility of providing a relatively unrestricted design for themask required for the fabrication method makes it possible to integratea completely novel aspect into the application of solar modulesfabricated in accordance with the invention. Because of their function,the solar modules are usually arranged in the visual range at any rate.It is now possible to utilize solar modules fabricated in accordancewith the invention as esthetic design elements for walls of buildingsand as advertising media, thus significantly increasing theirusefulness. Normally, the geometric pattern would consist of rectangularand straight-line narrow stripes. In order to keep them together withinthe mask, the mask may at its margins be provided with connecting ribs.During the application of individual layers for fabricating the solarmodule, the connecting ribs may be arranged outside of any given supportlayer. If for reasons of geometry this is not possible or where thegeometric pattern requires connecting ribs in its interior, particularlyto achieve sufficient mechanical sturdiness, such ribs would during theprocess sequence initially form jumpers. Depending upon the complexityof the geometric pattern it is thus useful, in accordance with animprovement of the inventive method, to provide for an additional methodstep following method steps (1.8) or (2.8): (A) Structuring shortcircuit areas in the front layer generated by connecting ribs in morecomplex geometric patterns.

The individual layers may be provided by generally known methods, suchas, for example, vapor deposition or sputtering. The composition of therequired layer package for a solar module in thin-film technologyfabricated in accordance with the invention arising is a function of thematerials used and of the applications. More particularly, prior toapplying the rear or front electrode in method step (1.2) or (2.1) thefollowing additional method step may optimally be provided in a furtheradvance of the invention: (B) Application of a barrier layer for forminga diffusion barrier. This may be a Cr layer which prevents aninterdiffusion, for instance, of Na. Furthermore, following applicationof the metal layer (1.3) or following structuring of the front layer(2.4) the following additional method step may be optimally provided:(C) Application of an adhesion and/or source layer for forming anadhesive agent. For instance, this may be a Na-source layer (NaF) and/oran adhesion-assist layer of, for instance, ZnSe or ZnS. Finally, anothermethod step may be optimally introduced between the absorber formingcoating and the application of the front layer, i.e. prior to methodstep (1.6) or (2.5): (D) Applying at least one buffer layer for forminga space-charge zone. This layer may consist, for instance, of CdS or ofZnS.

Depending upon the future application of the finished solar module, anapplication of transparent materials for forming the substrate orsuperstrate layers and/or the metal layer may be provided for as afurther advance of the method in accordance with the invention. Thiswould yield a special applicability for windows and semi-transparentareas and would take advantage of the fact that large-surface glasspanes make are usually used at any rate as substrates or superstratesfor thin-film solar cells. The material for forming the transparentmetal layer may, for instance, be ZnO, SnO or ITO (indium tin oxide),which in addition to other layers of different doping may also be usedfor forming the transparent conductive front layer (TLO). By contrast,non-transparent metall layers may consist, for instance, of molybdenum,wolfram (tungsten) or of another metal. Finally, for forming theabsorber layer, which also is non-transparent, a further advance of themethod in accordance with the invention may provide, depending upon thesupport layer, for the use of amorphous, poly- or monocrystallinesilicon, polycrystalline CdTe or chalcopyrite compounds of the generalformula Ag_(x)Cu_(1-x)In_(y)Ga_(1-y)S_(z)Se_(2-z-w)Te_(w) as thesemiconductor material, wherein x and y may assume values between 0 and1, and z and w may assume values between 0 and 2 such that the sum ofw+z does not substantially exceed 2. The mask may consist of differentmaterials which provide for the requisite mechanical sturdiness. Forforming substrate cells the mask may be made as a metal mask. Inprinciple, the mask need not be transparent as it will be covered by thenon-transparent absorber layer. A transparent but not necessarilymetallic mask may be used for fabricating superstrate cells if aseparate use of the mask is intended as a positive. However, beforereleasably fixing such a mask, which may consist, for instance, of glassor transparent plastic resistant to the method, on the superstrate, itmust first be prov at its upper surface with a transparent frontelectrode (TLO). Such coating may, for instance, be carried out in amanner similar to the coating for forming the rear electrode ofsubstrate cells.

The essential improvement and simplification of the method areaccomplished by the method in accordance with the invention by the useof the mask which may be structured in accordance with predeterminedwishes and marginal conditions. This allows elimination of a number ofstructuring processes as separate method steps. In particular, thelateral shifting of the mask eliminates two otherwise necessary scribingsteps. The measure of lateral shift is a spacer for covering from belowthe section of the electrode at the one side to provide access for thenext coating, and for covering from above at the other side to provide avoid from the next coating. The size of the covers from, below and fromabove relates to the overall dimensions of the structured solar cellsand is to ensure an assured cover on the one hand and an assuredseparation on the other side. In accordance with a further advance ofthe method is it thus advantageous laterally to shift the mask by about0.1 mm. Technically it is simple to execute and ensure such shifting,and it does not require any in the setup of the method betweenindividual method steps.

In the method according to the invention the mask assumes a significantrole at different aspects. Its direct coprocessing leads to no losses inmaterial. It is useful repeatedly to reuse masks, particularly those ofcomplex geometric structures and, therefore, higher costs, withoutintermediate reprocessing. Layers of insignificant material depositsapplied during prior method cycles do not interfere. When, in the end, amask is not used anymore, the applied material may be recycled which isof particular importance in large scale productions. In addition tothese and those advantages described supra the mask has the furtherinherent advantage that it may be used as a positive for its ownconfiguration, separate from the large-surface solar module which isconfigured quasi as a negative of the form of the mask. For that reason,in accordance with a further advance of the method in accordance withthe invention it is of overall advantage repeatedly to use the mask orfurther to process a removed mask the filled surfaces of which must thenbe configured to have uniform surfaces. This does not result in anothermethod sequence; processing on the substrate or superstrate and the maskcontinue identically. Since the mask may also be used no material islost at any step in the method and the relatively expensive material areoptimally used. Furthermore, the usability supports estheticconsiderations in which the geometric structures, in particular companylogos, may also be used as a positive. With structures of lessercomplexity, the mask may yield individual solar cells of simple geometrywhich by simple series connections may be combined to solar modules (seeinfra). However, in accordance with a further advance of the invention,care must be taken when defining the geometry that it provides for anesthetically and/or informally oriented geometric pattern of individualsolar cells adhering to partial patterns of identical surface size inits blank surface and/or in its filled surfaces. In this manner, thesolar cells on the negative as well as the solar cells on the positiveeach contribute an identical current thus avoiding the occurrence ofcurrent irregularities. It is not necessary that the surfaces of thefilled surfaces and of the blank surfaces be identical. In accordancewith a further aspect of the invention, additional design aspects may berealized by partial patters of different colors whereby the selectedcolors must be such that they can be integrated into the photovoltaicprocess.

In addition to esthetic considerations in solar modules fabricated inaccordance with the invention, the optimizing aspects regarding powermaximizing or surface minimizing referred to supra have to be taken intoconsideration. In an embodiment of the solar module fabricated inaccordance with the invention, it is particularly advantageous insupport layer types to provide a light-collecting concentrator moduleconsisting of individual concentrators as image-forming ornon-image-forming optical elements the arrangement of which matches thearrangement of the individual solar cells. The use of concentratorsmakes possible a significant increase of the average and total energyconversion efficiency of a solar module. The optical elements may, forinstance, be lenses of conventional semi-convex or Fresnel-likeconfiguration. In accordance with a further embodiment the solar modulemay at its light-impinging surface be encapsulated by a transparentglass or plastic material with or without transparent cover pane and theconcentrators are integrated in the glass or plastic or are applied toor ground into the interior surface of the cover pane. In particular,the application may be an adhesive one. Structuring of the exteriorsurface is disadvantageous, however, as it complicates cleaning and theeffect of weather and dirt may adversely affect the collecting action ofthe concentrators. Preferably, the concentrators used may have ageometric concentration factor of C_(g) which lies within a numericalrange between 1 and 10. Such concentrator modules are in principle wellknown and have been described in detail supra in connection with thestate of the art. But in connection with solar modules made inaccordance with the inventive method several interesting combinationsare possible. For instance, in accordance with a further embodiment ofthe invention, the concentrator may be arranged at a distance from thesolar module of solar cells structured as laterally straight lines inthe manner of a Venetian blind, the individual slats of which areconstituted by linear concentrator lenses which may be arranged to trackthe position of the sun. Such arrangements are particularly suitable formounting in a window, particularly those exposed to strong sunlight.Since the solar module itself may be made to be semi-transparent it maythus contribute to shadowing an interior room. An advantageousembodiment of the solar blind may be characterized by the fact that eachconcentrator lens is suspended at each end by two supports linked to twoguide rails which extend in guide slots fixed relative to the solarmodule in mounting blocks and may be adjusted simply by pressing movablewedge blocks. Thus all lenses may be adjusted in common. Furthermore,the concentrator lenses follow a path such that at different lightexposures their correct adjustment relative to the solar cells isensured.

In connection with the use of the mask as its own solar cell supportanother embodiment of the invention may provide for a solar moduleformed from the mask and for series connecting the electrodes of theindividual solar cells by way of an integrated metallized contactribbon. In such an arrangement, the contact ribbon may be structured asa transparent flexible contact foil the width of which corresponds tothe entire width of the solar module. In addition, the solar module maybe slidably mounted by lateral winding and unwinding of the contact foilextending beyond the solar module, in front of or behind (depending uponthe type of solar cell) a further structured and integratedseries-connected solar module which is mounted stationarily. Dependingupon the individual solar modules relative to each other a selection maybe made between maximum light transmissivity and maximum currentproduction. Such measures result in a partially transparent solar modulewith variable shadowing which may be structured according to estheticconsiderations. Overall, by an optimum combination of esthetic andfunctional structural elements, a solar module fabricated by the methodof the invention and modified optimally relative to its power output mayarranged by itself or in common with other solar modules in front of orwithin windows, wall and roof elements of a building, or in the interiorthereof. The degrees of transparency of the solar modules used may bematched with those of the building surfaces, and they may be changeable,for instance, fully transparent in front of windows disposed in theshade and in front structural elements made of glass, semi-transparentin front of windows exposed to the sun, and non-transparent in front ofbuilding walls, in roof areas and when used as sun screens. When used assemi-transparent solar modules, the surface to be covered by the solarcells may be significantly reduced by an application of concentrators.This leads to greater flexibility for architectural designs. The smalldistance between the solar and concentrator modules makes possible themanufacture of ready-to-use inserts used in the construction ofbuildings without requiring a significant increase in the space used forconventional solar modules. Completely new fields of application arebeing opened up which should make the use of solar modules—even withinbuilding—much more attractive. In order to avoid repetitions regardingthe mentioned modifications reference may be had to the followingspecific description for further details.

DESCRIPTION OF THE SEVERAL DRAWINGS

The novel features which are considered to be characteristic of theinvention are set forth with particularity in the appended claims. Theinvention itself, however, in respect of its structure, construction andlay-out as well as manufacturing techniques, together with other objectsand advantages thereof, will be best understood from the followingdescription of preferred embodiments when read in connection with theappended drawings, in which:

FIG. 1 depicts the sequence of the inventive fabrication method of asubstrate cell;

FIG. 2 depicts the sequence of the inventive fabrication method of asupertrate cell;

FIG. 3 depicts a solar module fabricated by the method and consisting ofsubstrate cells with a concentrator module;

FIGS. 4 a, 4 b depict a solar module in two positions, fabricated by themethod and consisting of substrate cells with a concentrator modulestructured in the manner of a Venetian blind;

FIGS. 5 a, 5 b depict in planar view and in section a solar modulefabricated by the method and consisting of substrate cells of variableshadowing; and

FIG. 6 depicts a diagram showing the effect of the concentrator modules.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 the method in accordance with the invention is depicted on thebasis of selected states of fabrication of an embodiment of a solarmodule to be fabricated shown in cross section. A thin-film mask 100corresponding to the desired geometry and adhering to the predeterminedmarginal conditions is fabricated in a first method step 1.1. Theembodiment shown is one of comb-like geometry having filled surfaces 101and blank surfaces 102 of identical size. Connecting ribs 103 in thegeometric pattern lie outside of the solar cell structure to befabricated and are not given further consideration. In a method step 1.2the mask 100 is releasably fixed on a transparent substrate layer 104 ofglass. In a next method step 1.3 a metal layer 105 is applied to thesubstrate layer 104 and the fixed mask 100. This leads to the formationon the substrate layer 104 of a rear electrode 106 structured in stripesshaped like the blank spaces 102 in the geometric pattern of the mask100. In method step 1.3 a metal layer 105 is also deposited on thefilled surfaces 105. In the following method step 1.4 the mask 100 islaterally shifted, for instance by about 0.1 mm, across the striped rearelectrode 106 in the direction of the arrow. At this point it is to bementioned that for the sake of a clear presentation the dimensions havebeen shown in distorted fashion. The lateral shifting leads to theformation of narrow lower cover ribs 107 and narrow upper cover ribs108.

A photovoltaically active thin semiconductor layer 109, for instance ofthe chalco-pyrite compound CuInS₂ is deposited in a next method step 1.5on the substrate 104 and the laterally shifted mask 100. This leads tothe formation of a structured absorber layer 110 over the mask 100 whichalso covers the lower cover rib 107; but it does not cover the uppercover rib 108. In this manner the striped rear electrode 106 in the areaof the lower cover rib 107 becomes encased by the semiconductor layer109 while remaining exposed in the area of the upper cover rib 108. Themask 100 is released and removed in method step 1.6. Thereafter, it maybe processed further separately but in parallel into a “positive solarmodule” and differs from the “negative solar module” only by the absenceof the substrate layer 104 which is, however, mechanically replaced bythe mask 100. A transparent conductive front layer 111 which forms thefront electrode 112 is applied in the following method step 1.7 on thesubstrate 104 in the area now vacated by the removal of the mask 100 andin the area of the upper cover rib 108 on the rear electrode 100 as wellas, but separate therefrom, on the removed mask 100. In this manner, theseparated mask 100 is fully processed and initially forms an unconnectedsolar module 113 of individual thin-film solar cells 114 in the form ofthe filled surfaces 101 of the geometric pattern without the connectingribs thereof. The solar module 113 may then be finishes (analogously inFIGS. 3 and 5) by subsequent suitable connection processes which may becarried out with relative ease and in integrated manner because of thelacking substrate layer 104 and the downwardly exposed metal layer 105.In the inserted state light impinges on the substrate 104 in thedirection of the arrow.

On the “negative” solar module 115 the front electrode 112 is initiallyunstructured and electrically short circuits all solar cells 116.Therefore, in an ensuing method step 1.8 the front layer 111 is thusopened in areas 119 of separation of the upper cover ribs 108 down tothe striped rear electrode 106 by a suitable scribing process, forinstance by a laser beam by gaps 117 which eliminate the short circuits.Generally the rear electrode 106 adheres more firmly to the substratelayer 104 than does the absorber layer 109. Thus, the solar cells 116are electrically series connected in an integrated manner with theindividual absorber layer strips 118 remaining electrically insulated.The solar module is thus finished and ready for use.

The inventive method of fabricating superstrate cells is analogouslyshown in FIG. 2. In a first method step 2.1 a thin mask 150 is againprovided in accordance with a predetermined geometric pattern. Ifsubsequent reuse of the mask is planned as a proper solar module, it ismade of a transparent material and on its upper surface it is providedwith a separately applied front electrode which has not been shown inthe figure. In an ensuing method step 2.2 a transparent conductive frontlayer 152 for forming a front electrode 153 is applied on a substrate151. In the embodiment selected the front electrode 153 consists of aplurality of SnO layers of different doping (ITO or ZnO are possible).Thereafter, the mask 150 is releasably fixed on the front layer 152(method step 2.3). Thereafter, in method step 2.4 the scribing of thefront layer 152 is performed along the outer margins of the mask 150functioning as mechanical or optical rulers, in order to structure thefront electrode 153. In a next method step 2.5 a semiconductor layer 154is applied for forming an absorber layer 155 structured incorrespondence with the geometry of the mask. In the manner of themethod described supra, the mask 150 is then, in method step 2.6laterally shifted by a small distance of about 0.1 mm. Thus, upper coverribs 156 and lower cover ribs 157 are formed. In the ensuing method step2.7 these are covered, exactly like the scribed structuring trenches158, by a metal layer 159 for structuring a rear electrode 160. In alast method step 2.8 the mask 150 is removed. The solar module isfinished with a corresponding structuring and integrated seriesconnection between individual thin-film solar cells 162. In an insertedstate light impinges through the superstrate 151 in the direction of thearrow.

Following the description of the two analogous methods of fabricatingsubstrate and superstrate cells, solar modules fabricated by the methodare to be described hereinafter in greater detail in connection with theuse of concentrator modules. This will always be based on the substratecell type. It is, however, expressly to be mention at this point thatall embodiments may also be executed on the basis of superstrate cellsafter appropriate conventional technical adjustment.

FIG. 3 (the meaning of reference numerals not explained here and in thefollowing figures may be taken from FIG. 1 or any preceding figure)depicts a solar module 200 fabricated by the inventive method, in apartially transparent embodiment with laterally structured solar cells2001 on a transparent substrate 202 and a light-collecting concentratormodule 203 as integrated light concentration. Such a solar module 200may be used, for instance, as a window or as an element of demandingarchitectural facades. In the selected embodiment, an enclosure whichrequired by every standard thin-film solar module in order to beprotected from weather conditions, is realized by a housing 204 whichalso serves to divert the generated current, and by a glass plate 205behind which is provided a transparent plastic 206 (e.g. epoxy orartificial resin) for filling the intermediate space. The concentratormodule 203 is arranged at the interior surface 207 of the glass plate205 and is provided with individual concentrators 208 which in theirarrangement are matched to the arrangement of the individual solar cells201. In the selected embodiment they are strip-like semi-convex lensesadhesively attached to the interior side of the glass plate 205.Reference should be had to FIG. 6 for an explanation of the effect ofthe concentrators.

FIG. 4 depicts an embodiment of a partially transparent solar module 300with solar cells 301 structured as straight lines on a transparentsubstrate 302 and provided with a concentrator module 303 structured asa solar Venetian blind including tracking linearly focusing concentratorlenses 304. The solar cells 301 are arranged behind the separatelysuspended blind-like concentrator module 303. This consists of as manylamellate linearly focusing concentrator lenses 304 as there arestrip-like solar cells 301 within the solar module 300. Eachconcentrator lens 304 is attached at its two ends by way of two shankedsuspension points 305 to two guide bars 306 which in turn extend throughguide slots 307 in mounting blocks 308. The position of the mountingblocks 308 is stationary with respect to the solar module 300 so that anindividual adjustment of the concentrator lenses 304 is avoided. Theguide bars 306 are adjusted in the guide slots 307 by simple pressure ofmovable wedge blocks 309. In this manner the concentrator lenses 304follow a path which ensure an appropriate adjustment relative to thesolar cells 301 at different incidents of light. The incident of lighthas been indicated by dash-dotted lines for two different angles ofincident in the upper (a) and in the lower portion (b) of FIG. 4. It isto be noted, firstly, that this kind of light concentration isparticularly suitable for superstrate cells in which the integration ofthe concentration into the solar module encounters difficulties and,secondly, that both the angle of inclination of the concentrator lenses304 and the position of their center of gravity are tracked correctly.The positioning signal for the tracking may be obtained in a simplemanner from the current output of the solar cells 301. In the presentembodiment the shape of the lateral pattern of the solar cells 301 hasto be a straight-line one so that proper irradiation by the concentratorlenses 304 may be ensured. It is possible, however, depending upon thedesired geometric concentration ratio to select a mark space ratiobetween solar cell and free space different from 2:1.

FIG. 5 depicts a partially transparent laterally structured combinationsolar module 400 with variable shadowing, in a planar view at (a) andcross-section at (b). The combination solar module 400 consists of astationary solar module 401 constructed of rigid solar cells 402 on atransparent substrate 403 and, arranged above the stationary solarmodule 401, of movable solar module 404 realized on a striped mask 405.Solar cells 406 prepared on the mask 405 are electricallyseries-connected to each other by a flexible transparent contact foil407 between the front and back sides of the striped solar cells 406. Thesurface of the contact foil 407 is metallized by a transparentconductive oxide. Thus, the contact foil 407 may be spread over theentire width of a window, thus resulting, on the one hand, in a lowseries resistance loss and, on the other hand, in increased mechanicalsturdiness of the flexible solar module 404. Each end of the connectingcontact foil 407 is wrapped over a cylindrical body 408 which alsoserves as outward electrical feed. The cylindrical body 408 is suspendedin a frame element 409 and may be rotated from the exterior so that theflexible solar module 404 may be moved laterally. In this manner, thestationary solar module 401 on the glass substrate 403 may beselectively shaded, rendering the window semi-transparent at low currentgeneration. Otherwise, the spatially variable solar cells 406 arepositioned between the rigid solar cells 402 of the stationary solarmodule 401 which renders the windows totally opaque and currentgeneration is maximized. In a variation of this embodiment it would notbe necessary to structure the solar cells 402, 406 as straight-linestripes; rather, they may also be structured according to estheticaspects as long as their surfaces satisfy the general marginalconditions and, for such application, are congruent as well. At thispoint it must also be mentioned that if superstrate cells were used themovable solar cells would have to be placed beneath the stationary solarcells.

Since the distance between the concentrators and the solar cells in theselected embodiments is relatively small, the concentrators will providefor low light concentration only, which in thin-film solar cellsnevertheless results in a significant improvement of the averageefficiency. FIG. 6 depicts a typical measuring curve of the efficiencyof a chalco-pyrite solar cell as a function of light concentration fromwhich three characteristic facts may be derived:

1. The average spectral irradiation energy at noon on a summer day isinternationally approached by the AM 1.5 global spectrum according toIEC norm 904-3 (1989) to be mentioned at this point. An our latitude,light conditions are prevailing in which the light intensity may vary byas much as factor 10 between summer and winter on the one hand and as aresult of variable cloudiness on the other hand. Hence, in its operationthe solar cell is subjected to an irradiation energy of about 10%–100%of the energy which in accordance with AM 1.5 is to be expectedglobally.

2. The energy efficiency relating to the irradiation intensity isvarying such that an operation at low irradiation levels is notfavorable. Thus, in the solar cell shown in FIG. 5 an efficiency ofη=9.2% at an irradiation of AM 1.5 global (C=1); however, at 10% of thatirradiation (C=0.1) the energy efficiency is only η=6.5%. The optimumefficiency of this solar cell, η=9.5%, is reached only at aconcentration of C=2−3.

3. By using light concentrators of low geometric concentration factorsC_(g) the irradiation level to be expected during operation is shiftedinto a range favorable to thin-film solar cells. FIG. 6 depicts threepossible concentration factors C_(g)=1, 3, 6, and it becomes apparentthat at a light incidence of 10%–100% of the standard sun AM 1.5 global,the average efficiency of this typical solar cell remains in excess ofη=8.8%, i.e. above 90% of the efficiency of 9.7% which may be achievedwith this cell, at a six-fold light concentration of C_(g)=6. A shift ofthe maximum to higher concentrations may be expected at an increasedoptimization of the properties of the solar cell (lowering of the seriesresistance).

Overall, the use of concentrators of a geometric concentration factorC_(g) in a numeric range between 1 and 10 is favorable. In most cases, ageometric concentration C_(g)=6 is entirely sufficient to ensure anoptimum efficiency. This low value renders the use of cost-efficientplastic Fresnel lenses in combination with chalco-pyrite cellsparticularly interesting. In such an arrangement, a solar cell typicallymeasuring from between 0.5–5.0 cm² rather than being placed preciselyinto the focal point of the Fresnel lens, is placed about 0.5 cm aheadof it thus rendering the illumination of solar cell homogeneous. Furthermeasures to increase the concentration such as, for instance, expensivesecondary concentrators of the kind necessary for Si or GaAs cells arenot necessary for amorphous and poly or microcrystalline solar cells.

1. A method of fabricating a solar module with structured and integratedseries-connected thin-film solar cells on a substrate; comprising thesequence of the following steps: (1.1) providing a thin-film maskaccording to a predetermined geometric pattern of filled surfaces withintegrated blank surfaces of uniform size; (1.2) releasably fixing themask on a substrate as support layer; (1.3) applying a metal layer forstructuring a rear electrode in the shape of the blank surfaces in thegeometric pattern of the mask; (1.4) laterally shifting the mask acrossthe structured rear electrode for forming narrow upper cover ribs in thedirection of the shift and lower cover ribs in a direction opposite theshift; (1.5) applying a photovoltaically active thin semiconductor layerof amorphous or poly or microcrystalline semiconductor material forforming a structured absorber layer; (1.6) releasing and removing themask; (1.7) applying a transparent conductive front layer of at leastone layer for forming a front electrode; and (1.8) structuring the frontlayer in separation areas of the upper cover ribs with short circuiteliminating separation gaps down to the metal layer of the rearelectrode.
 2. The method according to claim 1, further comprising theadditional step following step (1.8), of: (A) structuring short ciruitareas in the front layer generated by connecting ribs in complexgeometric patterns.
 3. The method according to claim 1, furthercomprising the additional step following step (1.2) of (B) applying abarrier layer for forming a diffusion barrier.
 4. The method accordingto claim 1, further comprising the additional step following step (1.3)of (C) applying an adhesion and/or source layer for forming an adhesiveagent.
 5. The method according to claim 1, further comprising theadditional step preceeding (1.6) of (D) applying at least one bufferlayer for forming a spatial charge zone.
 6. The method according toclaim 1, wherein transparent materials are used for forming at least oneof the substrate and metal layer.
 7. The method according to claim 1,characterized by a support layer dependent use of one of amorphoussilicon, polycrystalline silicon, microcrystalline silicon,polycrystalline CdTe and chalco-pyrite compounds of general formulaAg_(x)Cu_(1-x)In_(y)S_(z)Se_(2-z-w)Te_(w) as semiconductor material forforming the absorber layer, wherein x and y may assume values between 0and 1 and z and w values between 0 and 2 such that the sum of w+z doesnot significantly exceed the value of
 2. 8. The method of claim 1,wherein transparent materials are used for forming the mask.
 9. Themethod of claim 1, further comprising the step of laterally shifting themask by about 0.1 mm.
 10. The method of claim 1, further comprising thesteps of at least one of reusing and separately further processing thereleased mask and of forming the filled surfaces thereof to be of equalsize.
 11. A method of fabricating a solar module with structured andintegrated series-connected thin-film solar cells under a superstrate,comprising the sequence of the following steps: (2.1) providing athin-film mask according to a predetermined geometric pattern of filledsurfaces with integrated blank surfaces of uniform size; (2.2) applyinga transparent conductive front layer of at least one layer on asuperstrate as a support layer for forming a front electrode; (2.3)releasably fixing the mask on the transparent superstrate layer; (2.4)structuring the front layer along the filled surfaces in the geometricpattern of the mask down to the superstrate layer; (2.5) applying aphotovoltaically active thin semiconductor layer of amorphous or poly ormicrocrystalline semiconductor material for forming a structuredabsorber layer; (2.6) laterally shifting the mask over the structuredabsorber layer for forming narrow upper cover ribs in the direction ofthe shift and lower cover ribs in a direction opposite the shift; (2.7)applying a metal layer for structuring a rear electrode in the shape ofthe blank surfaces in the geometric pattern of the mask; and (2.8)releasing and removing the mask.